This invention relates to an apparatus for measuring torque through sensing shaft deflection. More particularly, this invention relates to an improvement over the torque measurement system disclosed in U.S. Pat. No. 3,548,649 which is assigned to the assignee of the present invention.
As disclosed in U.S. Pat. No. 3,548,649, the prior art has included systems for measuring torque through shaft deflection techniques. In such prior art techniques a torque shaft assembly uses two exciter wheels of magnetic material attached to a torque transmitting shaft at different axial positions thereon. As torque increases, the exciter wheels are rotationally deflected with respect to each other in a manner such that the rotational deflection is proportional to the applied torque. The torque sensor utilizes two magnetic pick-ups, each of which is mounted in close proximity to each of the exciter wheels, generating two approximate sinusoidal signals as the exciter wheel teeth pass the sensor pole pieces. The phase relationship of the two signals is related to the relative displacement of the exciter wheels, and is therefore related to the applied torque. Circuitry is provided to detect the phase relationship of the two signals and generate an indication of the torque.
As discussed in U.S. Pat. No. 3,548,649, such prior art systems were relatively complex and were subject to errors due to misalignment of a torque shaft with respect to the center line of the sensors. Accordingly, an improved system is disclosed in U.S. Pat. No. 3,548,649 which is of a "monopole" type, using only a single variable reluctance-type sensor.
In accordance with the specific disclosures of U.S. Pat. No. 3,548,649, there is provided a torque shaft assembly having two exciter wheels, one of which is a torque wheel attached directly to the shaft and the second a reference wheel attached to a reference sleeve which, in turn, is attached to the shaft spaced from the torque wheel. Each of the exciter wheels is provided with a row of axially extending tooth members along its periphery, and the two wheels are so positioned that the teeth from one wheel are positioned between the teeth of the other wheel. The distance that the teeth of one wheel extend into the space between the teeth of the other wheel is a sensing width used as a datum plane for a variable reluctance sensor positioned in close proximity thereto.
As the exciter wheels are rotated, an AC signal is produced by the sensor, wherein any two adjacent cycles of the signal will be controlled by the relative distance between the teeth in the sensing width depending upon the relative position between the two wheels. A zero cross-over detector converts the sensor AC signal into a train of pulses which are utilized to alternatively turn "off" and "on" a bistable multivibrator, producing a rectangular waveform whose ratio of "on" time to "off" time is directly related to wheel tooth position. The bistable multivibrator output is filtered into two DC voltages, the greater positive output of which is carried to the input of a display portion of the system.
Torque measurement systems constructed in accordance with the teachings of U.S. Pat. No. 3,548,649 have received wide commercial acceptance. However, systems such as disclosed in U.S. Pat. No. 3,548,649 did not have any means for indexing the system, so that there could be a potential ambiguity with respect to the output signals, as to which output signal corresponded to which of the alternate spacings between the adjacent teeth. Accordingly, in such systems the exciter wheels were mounted with respect to each other such that there were equal distances between the teeth of the two exciter wheels in the no-load condition. Under load conditions the tooth spacing of a tooth of the torque exciter wheel to an adjacent tooth of a reference exciter wheel in the direction of torque deflection would always be smaller than the spacing between a tooth of the reference wheel and an adjacent tooth of the torque wheel in the direction of torque deflection. Thus, potential ambiguity in outputs in the loaded condition is eliminated. However, in the system of U.S. Pat. No. 3,548,649, due to the no-load symmetry in the tooth spacing between the two exciter wheels, the full torque situation represented by a deflection between the two exciter wheels of only less than one half of a tooth space. Generally, such systems used a deflection of one quarter of one tooth to tooth spacing for full torque, corresponding to an electrical phase range of 20.degree.-50.degree..